Well the easiest way is a humidifier, and should really be part of your set up. Other than that, you would need to try and use the air outside to raise it. What are they in a tent? You could use a plain water foliar sprayer around the area not directly on the plants to raise humidity. Hang wet towels near them ect. You could also check your RH outside and have that air flow into the grow area but this could introduce another set of problems, pests, pathogens ect.
Don't worry about them too much, just ph as best you can for now and make that folair spray, nice and weak, once like I said and maybe again before lights off.
So 1x plain water foliar spray not directly on the plants to raise humidity
1x 150ppm PH'd foliar spray directly on plants 30 mins before lights on, don't soak them too much, just a nice misting will do.